1. Field of the Invention
The present invention relates to a fuel injection quantity control device which is capable of suppressing overshoot and undershoot when the actual revolution speed of an engine is controlled to the target revolution speed.
2. Description of the Related Art
When the actual revolution speed (rpm) of an engine is controlled to the target revolution speed (rpm), the control is conducted so as to increase or decrease the fuel injection quantity. The inventors are presently developing the following procedure for computing the fuel injection quantity.
This procedure comprises the steps of finding a difference e by subtracting the actual revolution speed from the target revolution speed, finding the proportional term output value (Qp=Kpxc2x7e) by multiplying the difference e by the prescribed proportionality constant Kp, finding the integral term output value (Qi=∫(Kixc2x7e)dt) by integrating the product of difference e and the prescribed integration constant Ki, and obtaining the final injection quantity by adding up those proportional term output value Qp and integral term output value Qi. With this procedure, because not only the proportional term output value Qp but also the integral term output value Qi is used, the speed response is good.
Japanese Patent Application Laid-open No. H4-134155 is known as a reference relating to pertinent conventional technology.
However, with the above-described procedure, for example, when the actual revolution speed is brought up to the target revolution speed, that is when the difference is positive, the difference is continued to be added up in the process for computing the integral term output value till the difference between the two speeds becomes 0. Therefore, in the point of time at which the difference becomes 0, the fuel injection quantity can become too large causing overshoot (the actual revolution speed is above the target revolution speed).
Conversely, when the actual revolution speed is brought down to the target revolution speed, that is, when the difference is negative, the difference is continued to be subtracted in the process for computing the integral term output value till the difference between the two speeds becomes 0. Therefore, in the point of time at which the difference becomes 0, the fuel injection quantity can become too small causing undershoot (the actual revolution speed is less than the target revolution speed).
It is an object of the present invention, which was created with the foregoing in view, to provide a fuel injection quantity control device which is capable of suppressing overshoot and undershoot when the actual revolution speed of an engine is controlled to the target revolution speed.
In order to attain the above-described object, the present invention provides a fuel injection quantity control device for controlling an actual revolution speed of the engine to a target revolution speed, comprising: difference computation means for subtracting the actual revolution speed from the target revolution speed and finding the difference therebetween; proportional term computation means for multiplying the aforesaid difference by the prescribed proportionality constant and finding a proportional term output value; integral term computation means for finding an integral term output-value which is obtained by integrating the product of the aforesaid difference and the prescribed integration constant; differential term computation means for finding a differential term output value which is obtained by multiplying the value obtained by differentiating the aforesaid difference by the prescribed differentiation constant; and injection quantity computation means for adding up the aforesaid proportional term output value and integral term output value and determining the injection quantity, wherein the device further comprises correction means for limiting the lower limit of the integral term output value with the differential term output value when the aforesaid difference is negative, thereby suppressing the excess reduction of the injection quantity, and limiting the upper limit of the integral term output value with the differential term output value when the difference is positive, thereby suppressing the excess increase of the injection quantity.
With the fuel injection quantity control device in accordance with the present invention, when the actual revolution speed of the engine is controlled to the target revolution speed, overshoot and undershoot can be suppressed. Thus, limiting the lower limit of the integral term output value Qi with the differential term output value Qd suppresses undershoot, and limiting the upper limit of the integral term output value Qi with the differential term output value Qd suppresses overshoot.